1. Field Of The Invention
This invention relates to gas detectors and, more particularly, to a gas detecting apparatus which can selectively monitor one of a plurality of gases in a sampled atmosphere.
2. Description Of The Prior Art
It is often desirable to measure the concentration of gas levels, such as oxygen or combustible gases, in a sampled or confined atmosphere. Low levels of oxygen present a danger to human life. For combustible gases, typically one is concerned with the lower explosive limit, the lowest concentration at which a flame will propagate or burn when the gas is exposed to an open flame or spark. In addition, inert and other gases also may pose a hazard as they displace oxygen in the air.
A number of methods have been developed for detecting specific gases. For example, the use of catalytic sensors to measure the concentration of combustible gases is well known. In such a sensor, a resistive element is coated with a catalytic material which reacts when contacted with a particular gas. A current passing through the resistive element heats the catalytic coating to a desired operating temperature and as the gas and oxygen contact the catalytic coating, it chemically reacts thereupon and adds heat to the resistive element. This additional heat changes the resistance of the coated element, a change that can be detected electrically in various known ways. In a common arrangement, a catalytic sensor, combining an active element in series with a reference element, is provided in one branch of a Wheatstone bridge. Voltage imbalances in the Wheatstone bridge are measured and provide a reading of the concentration of the gas contacting the sensor. Other known sensors for detecting a combustible gas include metal oxide semiconductor sensors in which the conductivity of the material changes when contacted by a particular gas. This change in conductivity can also be measured electrically to provide a direct reading of the concentration of the gas.
For many known sensors, the electrical signal generated when the sensor is contacted by a particular gas is proportionally related to the gas concentration. The slope of a plot of gas concentrations versus electrical activity in the sensor is often referred to as a sensitivity for the sensor. The ratio of the sensitivity of a sensor to different gases is sometimes referred to as a K factor, conversion factor, scaling factor or multiplier. Once the K factor for a gas is calculated or determined empirically, unknown concentrations of gas can be determined by measuring the electrical response of the sensor, such as a voltage change, and multiplying that response by the K factor for the sensor for that gas. This method allows the user to measure concentrations of various gases with an instrument that has been calibrated with a single gas, e.g., measuring hydrogen with an instrument calibrated on methane. While it is possible to use several K factors over a range of electrical activities, representing non-linearities in the gas sensor response, it is more common to use a single K factor for the entire range of operation of the sensor.
Gas sensors are typically "tuned" or constructed to react with and generate electrical responses relative to a single, predetermined gas. Measurements involving pluralities of unknown gases often use multiple sensors, each of which is tuned to a particular gas, or use complicated arrays or networks of sensors in conjunction with analysis equipment to generate readings for various gases. An example of a prior art arrangement for detecting the level of a combustible gas, the level of oxygen, and the displacement of air by an unknown third gas is shown in U.S. Pat. No. 4,664,886. Other arrangements for detecting multiple gases are shown in U.S. Pat. Nos. 4,481,804, 4,542,640, 4,567,475, 4,670,405, 4,818,348, 4,847,783, 5,007,283, and 5,025,653.
It is often desired to measure one of a plurality of gases, such as 10-15 gases, in a sample atmosphere. The prior art arrangements require the sample atmosphere to be monitored by a plurality of expensive gas sensors, or require that a gas sensor be changed each time a different gas present in the sample atmosphere is to be detected.
It is an object of the present invention to provide a gas detecting apparatus which can readily and selectively measure one of a plurality of gases at unknown concentration levels. It is an object of the present invention to provide such a gas detector which is easy to use, easy to select from one gas to another, and which is not overly complicated or expensive to manufacture. It is also an object of the present invention to provide such an arrangement without using a plurality of separate gas sensors.